OP here! I did follow store policy. People could not go to our restroom. That’s because the restroom is inside the stock, next to our safe. We could not keep an eye on them, so it is possible that they could steal from us. Althought we would never refuse a child or a pregnant woman.
This lady took of her pants and underpants. When she left the fitting room, she acted like nothing happend. We did give her something to clean it up.

In what world is peeing on the fitting room discreet?! It's pretty easy to tell what kind of clothing stores have restrooms, and if you're the kind of person who needs to get have quick access to a bathroom to pee, you pay attention to these things when you're shopping. She should have been able to get to another location. However, if she truly has real incontinence issues, frankly, it IS more discreet to pee in your pants. If it showed, hell, she's in a clothing store - she could probably buy a pair of bottoms and even underwear right there! People should have some decency not to make others deal with their messes!

Peeing in the fitting room is gross, but I never really understood "rules" surrounding the use of restrooms in public places of business. If you invited somebody into your home, and they needed to use the restroom, you would let them. This is no different. By turning on the "open" sign, you are inviting guests into your business. It just seems so petty not to allow somebody to use the facilities if they need to.

Unfortunately, not all stores have a bathroom that is available to the public.
I used to work at a smallish bookstore that had a tiny bathroom; it could only be accessed by going *behind* the sales counter, down a narrow and curved set of stairs, and into the basement stock area. Definitely NOT a place we wanted customers to be going.